The transmission-side circuit arrangement of a mobile terminal apparatus based on a general W-CDMA scheme will be described with reference to FIG. 9. A baseband signal comprises an in-phase component (to be referred to as an I signal) and a quadrature component (to be referred to as a Q signal) in quadrature modulation. A digital baseband unit 112 generates this signal. RRC (Raised Root Cosine) roll-off filters 110 and 111 for waveform shaping band-limit the I and Q signals. The processing so far is digital signal processing.
D/A converters 108 and 109 then respectively convert the I and Q signals into analog signals. A known quadrature modulator 106 performs quadrature modulation for a local signal with these analog signals. The high-frequency signal generated as a result of this operation is input to a variable gain amplifier (VGA) 105, which then amplifies it to a predetermined level in accordance with the gain control signal output from the digital baseband unit 112 or the analog signal converted from the gain control signal by a D/A converter 107.
The high-frequency signal amplified by the variable gain amplifier 105 contains many spurious components. A bandpass filter (BPF) 104 removes these spurious components. The resultant high-frequency signal is then amplified by a power amplifier (PA) 102 and transmitted from an antenna 101. Although a power supply 103 drives the power amplifier 102, FIG. 9 shows the voltage of the power supply 103 as a fixed voltage.
FIG. 10 shows the arrangement of a circuit for generating a baseband signal in a currently commercialized W-CDMA (referred to as R99: Release 99) mobile terminal apparatus. Reference symbol DPCCH denotes a control channel, which is a binary signal of ±1. A multiplier 133 multiplies this signal by a spreading code Cc (which is also a binary signal of ±1). A multiplier 134 then multiplies the signal by a weighting factor βc. On the other hand, reference symbol DPDCH denotes a data channel, which is a binary signal of ±1 as in the case with DPCCH. A multiplier 130 multiplies this signal by a spreading code Cd (which is also a binary signal of ±1). A multiplier 131 then multiplies the resultant signal by a weighting factor βd.
In the R99 system, a baseband signal comprises only these two-system signals in reality. A scrambler 138 multiplies this signal by a scramble code, and then outputs real and imaginary parts as I and Q signals, respectively. Reference numerals 132 and 135 denote combiners; 136, a multiplier which multiplies j representing an imaginary number; and 137, an adder which adds a real part and an imaginary part.
FIG. 11A shows the constellation of baseband signals (loci on an IQ plane) after they pass through the RRC roll-off filters 110 and 111. Referring to FIG. 11A, the ratios between the values of weighting factors β are βc=8/15 and βd=15/15. In the constellation chart, the white dotted line circle is a circle whose radius is defined by the RMS (root mean square) value of signal amplitudes, and the black solid line circle is a circle whose radius is defined by a peak value. According to R99, since the number of code channels constituting a baseband is only two, i.e., DPCCH and DPDCH, the ratio (PAR: Peak Average Ratio) between the peak value and the RMS value is small. When this value is represented by dB, the resultant value is about 3.3 dB at most.
FIG. 12 shows the arrangement of a circuit which generates a baseband signal based on an HSDPA (High Speed Downlink Packet Access) (R5: Release 5) scheme which is expected to be commercialized in the near future. The same reference numerals as in FIG. 10 denote the same parts in FIG. 12. In Release 5, HS(High Speed)-DPCCH, which is a new control channel, is additionally provided as a response channel for a high speed downlink data channel, as shown in FIG. 12. An HS-DPCCH signal is an uplink control channel for HSDPA, and is a binary signal of ±1. A multiplier 139 multiplies this signal by a spreading code Chs (which is also a binary signal of ±1). A multiplier 140 multiplies the resultant signal by a weighting factor βhs. With the addition of this HS-DPCCH, the PAR increases to about 5 dB.
According to HSUPA (High Speed Uplink Packet Access) (R6: Release 6) which is expected to be adopted in the future, the number of code channels greatly increases, as shown in FIG. 13. The same reference numerals as in FIG. 12 denote the same parts in FIG. 13. In addition to DPDCH, high-speed data channels E-DPDCH1 to E-DPDCH4 are additionally provided, which are respectively spread by unique spreading codes Ced,1 to Ced,4 (multipliers 141, 143, 145, and 147) and respectively weighted by unique weighting factors βed,1 to βed,4 (multipliers 142, 144, 146, and 148).
In addition, a control channel E-DPCCH for controlling these communications is additionally provided. The control channel E-DPCCH is spread by a unique spreading code Cec (a multiplier 149) and weighted by a unique weighting factor βec (a multiplier 150). FIG. 11B shows a constellation in a case in which the ratios between weighting factors are βc=βhs=βec=8/15, βd=0, βed1=βed2=15/15, and βed3=βed4=11/15. Compared with R99, the gap between the peak value and the RMS value is large, and the PAR is about 7 dB. That is, the PAR is larger than that in R99 by as large as 4.6 dB.
Compared with R99, therefore, HSUPA cannot meet the adjacent channel leakage power standard when amplification is performed by the same amplifier, because a large distortion occurs at an amplitude peak, unless the transmission power is decreased. A dB value indicating how much the transmission power should be decreased to meet the adjacent channel leakage power standard is called a back-off. Since R99 is currently in practical use, a dB value indicating how much the transmission power is decreased as compared with R99 to obtain the same adjacent channel leakage power as that in R99 is called a back-off relative to R99. This value will be simply referred to as a back-off hereinafter.